The present invention relates to forming a thin film product, and particularly to a thin film product in which a thin film structure is disposed on a patterned substrate having nanoscale nucleation sites, where the thin film structure and the patterned substrate are heterogeneous, i.e. they are composed of different materials.
In forming an electronic workpiece (e.g. of the type which is useful as an electronic device, as an optoelectronic device, etc) it is known to form a thin film product by growing one or several epitaxial layers (epilayers) onto a substrate. The thin film product is processed (e.g. by the formation of electronic components and circuits) to create a particular type of electronic workpiece. When a thin film product is formed with an epilayer having the same crystalline orientation as that of the substrate onto which the epilayer is grown, the epilayer is referred to as an xe2x80x9cepitaxial layerxe2x80x9d and the process of growing the epilayer on the substrate is referred to as xe2x80x9cepitaxialxe2x80x9d film growth.
Epitaxial film growth has progressed from homoepitaxy (epilayer material same as substrate material) to lattice matched heteroepitaxy (epilayer material different from substrate material but lattice structure and spacing of film very close to that of substrate) to pseudomorphic, lattice mismatched heteroepitaxy (epilayer material different from substrate material, lattice spacing of epilayer different from lattice spacing of substrate, and strain accommodated in very thin films forming at least part of the epilayer).
A problem which has been recognized in connection with lattice mismatched heteroepitaxy has been the effect of material defects such as dislocations and stacking faults due to mismatch stress on the electronic and optical characteristics of the final devices and circuits. Heretofore, a problem has been that the mismatch stress present at the substrate/epilayer interface remains constant throughout the film as growth proceeds and consequently the strain energy grows linearly with epilayer thickness. Dislocations are then eventually created in the epilayer as the integrated strain energy becomes larger than the energy required to nucleate a defect. This has limited defect-free, heterogeneous film growth to very thin layers below the critical thickness at which dislocations form. Often these thicknesses are not sufficient for optimizing device properties.
Theoretical work reported in 1986 by Luryi and Suhir in xe2x80x9cApplied Physics Lettersxe2x80x9d [New Approach to the High Quality Epitaxial Growth of Lattice-Mismatched Materials, Appl. Phys. Lett. Vol. 49, pp. 140-142 (1986)] suggested that when epitaxial growth was restricted to small, isolated areas of patterned substrates, a higher degree of substrate lattice mismatch could be accommodated in an epilayer, and that defect-free growth thicknesses larger than possible for large area growth could be achieved.
In addition, some of the problems discussed above in connection with epitaxial structures and products can also be encountered in forming non epitaxial structures and products by techniques in which thin film structures are deposited on heterogeneous patterned substrates having nanoscale nucleation sites and it is desirable to reduce strain due to differences in thermal expansion coefficients of the thin film structures and the patterned substrates.
One aspect of the present invention relates to a new and useful application of nanoheteroepitaxy for the growth of thin films in a manner which further reduces strain at the substrate-epilayer interface. An epilayer is grown on a substrate by: (a) providing a patterned substrate comprising a plurality of discrete, nanoscale islands (xe2x80x9cnucleation sitesxe2x80x9d), and (b) selectively growing an epilayer atop each of these islands but not in the spaces between the islands, in a manner which localizes and apportions strain at the substrate-epilayer interface, and enables strain to reduce as the thickness of the epilayer increases. Depending on the specific material parameters, strain-related defects are either completely eliminated or confined to the same inactive region near the substrate-epilayer interface. The strain is shared between the substrate and the epilayer rather than being confined almost exclusively to the epilayer as in the Luryi and Suhir procedure. This enables an epilayer which is substantially defect free to be grown to thicknesses well beyond that predicted by Luryi and Suhir for identically sized epitaxial nucleation island regions.
An additional aspect of the present invention is the use of active compliance, wherein advantage is taken of the modification of elastic moduli in nanoscale materials.
The principles of the present invention are particularly effective at growing an epilayer on a heterogeneous substrate.
Moreover, the principles of the present invention are believed to have more general application in forming thin film structures on heterogeneous patterned substrates having nanoscale nucleation sites, where it is desirable to reduce strain due to differences in thermal expansion coefficients of the thin film structures and the patterned substrates.
Still further, with the principles of the present invention, an electronic and/or optoelectronic workpiece can be created in which strain defects are localized at an inactive region of the electronic and/or optoelectronic workpiece, and their density decreases with increasing epilayer thickness, so that the active regions of the epilayer are substantially defect free.
In this application, reference to xe2x80x9cnanosizedxe2x80x9d islands (or nucleation sites) is intended to mean linear dimensions less than xcx9c100 nm, and a xe2x80x9cnanosized epilayerxe2x80x9d is intended to mean an isolated epilayer segment grown substantially atop a single nanosized island. Moreover, the concept of xe2x80x9csubstantiallyxe2x80x9d reduced strain in an epilayer or an epilayer being xe2x80x9csubstantiallyxe2x80x9d defect free is intended to mean a layer with sufficiently low defect density that it is usable for device applications. The specific levels required vary from device to device and are a function of many parameters and aspects of the device performance which differ between applications.
Further features of the present invention will become further apparent from the following detailed description and the accompanying drawings.